The purpose of this study is to evaluate a new experimental medication, UT-15, to treat pulmonary hypertension. This medication is given continuously, 24 hours a day, subcutaneously in the abdomen, and is delivered by a small pump. Previous pilot studies have demonstrated similar hemodynamic effects to prostacyclin, the only approved therapy for patients with primary pulmonary hypertension. Although prostacyclin has not been approved for the treatment of patients with pulmonary hypertension who also have congenital heart disease, as well as connective tissue diseases, we and others have demonstrated the safety and efficacy of prostacyclin in these groups of patients with pulmonary hypertension as well. UT-15 is chemically more stable and, by chronic subcutaneous administration, should significantly decrease the complications and side effects due to the delivery system of chronic prostacyclin, which unfortunately can only be given by intravenous infusion 24 hours a day. The study design is a 12-week multicenter, double-blind, randomized, placebo-controlled study to assess the safety and efficacy of chronic subcutaneous UT-15 plus conventional therapy to conventional therapy alone in 224 patient with symptoms of pulmonary hypertension.